1. Field of the Invention
The present invention relates to a matrix substrate, a liquid crystal device employing the matrix substrate, and a display apparatus employing the liquid crystal device for displaying pictures and characters. More specifically, the present invention relates to a matrix substrate characterized by a means for supplying a video signal to a vertical signal lines of a horizontal-vertical scanning circuit for driving a liquid crystal device, a liquid crystal device employing the matrix substrate, and a display apparatus employing the liquid crystal device.
2. Related Background Art
In the present multimedia age, communication apparatuses using picture information are becoming more and more important. Of the communication apparatuses, liquid crystal display apparatuses are attracting attention because of the smaller thickness and the less power consumption, and the liquid crystal production is growing up to a basic industry comparable to the semiconductor industry. The liquid crystal displays are employed mainly to 10-inch note-sized personal computers at the moment. The liquid crystal displays are considered to be useful also for work station displays, home televisions, and other larger size display apparatuses. However, with the increase of the picture size, the display manufacture equipment becomes expensive, and display apparatus requires high levels of the electric properties for driving the larger picture display. With the increase of the picture size, the manufacture cost increases rapidly in proportion to 2-3th power of the picture size.
In recent years, to solve the above problems, a projection system is attracting attention in which a picture image formed by a small liquid crystal display panel is enlarged by optical projection. This system enables improvement of the characteristics and decrease of the production cost by size reduction similarly as the scaling law which teaches improvement of properties and cost by a finer semiconductor. The TFT type liquid crystal display panel requires small TFTs having sufficient driving force, and the TFTs are shifting from amorphous silicon type to polycrystalline silicon type. The picture signal of the resolution level of the NTSC Standard for usual television does not require so high speed treatment.
Therefore, not only TFT but peripheral driving circuit such as a shift register and a decoder is produced by use of polycrystalline silicon to provide a liquid crystal display apparatus having the display region and the peripheral circuit in integration. However, the polycrystalline silicon does not comes up with the monocrystalline silicon, and the shift register and other devices employing polycrystalline silicon have to be divided into portions to provide a high-quality television having a resolution level higher than NTSC Standard, or to realize a display of XGA (extended graphics array) level or SXGA (super-extended graphics array) level of computer resolution standard. Such division of the shift register or the like causes noises called a ghost in the display region corresponding the joint portions. This noise should be prevented.
On the other hand, the display apparatus employing a monocrystalline silicon substrate giving a remarkably high driving force is attracting attention in comparison with the polycrystalline display apparatus of an integration structure. This display apparatus employing the monocrystalline silicon substrate need not be divided as above since the transistors of the peripheral driving circuits have sufficient driving force, and have high S/N ratios at the joint portions of the display apparatus and the peripheral driving circuits without the noise and other disadvantages.
By use of either the polycrystalline silicon or the single crystalline silicon, a reflection type liquid crystal device can be provided by incorporation of a reflection type liquid crystal element prepared by connecting a TFT drain with a reflection electrode, and holding a liquid crystal between the reflection electrode and a light-transmissive common electrode.
The scanning circuit is also important which conducts displaying by writing video signals to respective picture elements of the reflection type liquid crystal element. An example thereof is explained by reference to FIG. 40. In FIG. 40, digital image signals are converted by signal processing circuit 230 (D/A conversion), and are inputted as analog signals to the chips of a liquid crystal device. Horizontal scanning circuit 231 inputs its control signals to the gate of transfer switch 234. In accordance with the control signal, the analog signals on common signal line 238 are led out sequentially to respective vertical signal lines 232. Vertical scanning circuit 950 outputs sequentially control signals to scanning lines 233. Scanning lines 233 are connected to gates of switch MOS transistor 235 in the liquid crystal picture elements. The sources thereof are connected to vertical signal lines 232. When switch MOS transistor 235 is turned on, the video signals at vertical signal line 232 apply electric field to liquid crystal 237 to keep a potential of additional capacity 236 to hold display.
The above described prior art technique has disadvantages below. Generally, the rate of writing is extremely high, so that the analog signals are formed as high frequency signals. Therefore, the transfer switch is required to conduct the transfer at an extremely high rate, which necessitates a larger size of the transfer switch. Furthermore, the wiring is necessary for connecting the above common signal lines to pads for connection to an external circuit, which results in a large capacitance of the common signal lines. In the liquid crystal display element, the amplitude of the analog picture signals is as high as 10 V or more, which requires a high performance of the external driving circuit for driving the load of high capacitance at a large amplitude at a high speed, and requires a high power consumption, disadvantageously. If a flexible cable or the like is used for connection to the external circuit, the reactance component is accompanied in proportion to the cable length, causing larger ringing of the signal with a larger load capacitance of the common signal lines. Therefore, the length of the cable is limited which reduces mechanical freedom disadvantageously.
Generally, with increase of number of display picture elements, driving speed of signal lines connected to transistors on an active matrix substrate has to increase. Japanese Patent Laid-Open Application No. 2-216190 (JPA02-216190) discloses a method of reducing the driving speed. This disclosed invention employs, as a signal line driving circuit element, a D/A converter which is constituted at least of a capacitor and a switch and receives signals of plural bits of gradation information, and inputs the plural bits of signals in series with time. This disclosure is explained by reference to FIG. 41.
In FIG. 41, the numeral 1030 denotes a liquid crystal panel employing a thin film transistor (not shown in the drawing) as a switching element. Thereto, signal line driving circuit 1040 and scanning line driving circuit 1050 are connected. Signal line driving circuit 1040 is constituted of shift register 1041, latch 1042, D/A converters 1043, and switches S1-Sn. In this display apparatus, the data of one-scanning electrodes are inputted to shift register 1041. The data are transferred to latch 1042 capable of holding data of 7-bit breadth, and held there. The data are then inputted to switched capacitor/series conversion type D/A converter group 1043. In D/A converters 1043, the data of the 7-bit breadth are inputted, bit by bit, successively and seven times of conversion is conducted to output analog signals. In this device, the D/A converter is employed for inputting digital signals. In the display device, the D/A conversion is reported to be conducted with high precision with a small circuit size since the D/A converter is constituted of a capacitor, a switch, and the like, and the D/A conversion is conducted by inputting bit data in series with time.
However, the liquid crystal devices for XGA, SXGA, or the like having a large number of display picture elements require the D/A converters corresponding to the number of the signal lines in the periphery of the display region of the liquid crystal for introducing the analog signals to the signal lines. This is not practical.